In order to measure radiation exposure of personnel or environmental areas, a thermoluminescence dosimeter is frequently carried by the personnel or stationed in the area to measure accumulated radiation dosage. The dosimeter contains a crystalline material that traps some of the electrons released during exposure to radiation.
Frequently, the detector element in a thermoluminescence dosimeter is a small chip of white, crystalline calcium fluoride. This material has the property of giving off light in proportion to the accumulated ionizing effects (or dose) of radiation incident thereon. Light is emitted when the calcium fluoride is heated to approximately 300.degree. C. The quantity or peak intensity of the light emission ("thermoluminescence") is directly proportional to the radiation dose accumulated since the last heating cycle ("readout").
By continuing heating to approximately 600.degree., most of the electrons caught in the higher energy traps in the crystal are "baked out" to minimize their subsequent escape to lower energy traps. This results in a stable "zero" condition when the detector element is subsequently cooled to room temperature. This zero condition continues until ionizing radiation exposure in the crystal dislocates electrons which are trapped in the crystal. The dislocated electrons accumulate until released during a subsequent heating cycle wherein light is again emitted as the electrons return to the normal energy level.
It has been observed from periodic thermoluminescence dosimeter recalibration data and special testing that the thermoluminescence dosimeter readings for the same exposure decrease exponentially as a function of the number of times a thermoluminescence dosimeter is used. Thus, the reuseability of a thermoluminescence dosimeter is defined as the number of times the dosimeter can be used (i.e., the number of readouts) before its light output reading declines by more than 15% from the standard for the same exposure. Although presently available commercial units maintain the required sensitivity for 300 use cycles, the potential for a 15% error in sensitivity, in addition to other experimental and statistical errors inherent in the use of the units, is considered undesirable.
The use of a calcium fluoride detector element in a thermoluminescence dosimeter is disclosed in U.S. Pat. No. 4,039,834 (Lucas et al). According to the specification in this patent, the calcium fluoride material is relatively stable in humid air so that the manner in which the detector chip is mounted or encapsulated is not critical. It is suggested in this patent that optimally, the chip should be encapsulated in a sealed enclosure containing an inert gas such as nitrogen or argon.
Other patents of general interest, which disclose various forms of dosimeters, include the following: U.S. Pat. Nos. 3,371,209 (Rutland et al); 3,789,227 (Eichner et al); 4,224,519 (Gammage); and 4,105,918 (Miyagawa et al). The first three patents disclose the use of calcium fluoride as a detector element and the last patent discloses the use of calcium sulfate.